As long as you know how to use Notepad or Notepad++ (I prefer using this one), then you can edit your website template in any operating system/computer you have. -cypher.
All the templates can be edited with any operating system installed, Are you talking about a particular website designing framework?
NOT that 'templates' are a good way to deploy a site in the first place -- since you'd be shoehorning content into a layout that it might not be meant for -- it's why I say content should dictate markup/layout and not the other way around. It's another of the sleazy shortcuts and broken methodologies that just continues to increase my disgust with the industry as a whole.
There are some website which are selling templates you can by templates from there, sites are templatemonster dot com, themeforest dot net any many more